The present invention relates to the production of optical fibers and more specifically to an improvement made to the injection of the gases used during this production process.
The term optical fiber is generally understood to mean a light wave guide which, in its simplest form, consists of at least two layers of glass. One layer forms the core of the fiber and the other forms the fiber cladding and is placed round the core, whilst having a refractive index below that of the core. In certain applications, it is known to use a fiber in which the index varies radially in a continuous or discrete manner from a higher value at the centre to a lower value at the periphery.
Various processes are known for the production of such graded-index fibers and they generally consist of three fundamental steps:
the production of a solid glass cylinder, called the preform, whose index varies discretely or continuously from the centre towards the periphery;
fiber formation, i.e. the drawing out of the preform in order to obtain a greater fiber length, whose diameter is well below that of the preform; and
the protection of the thus obtained fibers and their insertion into cable structures.
A number of different processes for producing the preform are also known and in particular a vapour phase deposition process known under the initials M.C.V.D. (modified chemical vapour deposition) making it possible to obtain high quality graded index fibers. This process consists of using a hollow cylindrical silica tube traversed by a gaseous stream of silicon chloride, oxygen and a certain number of compounds (generally halides) of materials for doping the silica, in order to vary its index. The tube is externally and locally heating to bring about an oxidation of the gases and the deposition and vitrification of the products of the reaction within the tube and specifically at the heated point. The heating zone is moved at constant speed along the tube. As the thickness of the deposit obtained during each passage is very low, numerous passages (roughly at least 100) are required, bearing in mind that for each of these processes it is necessary to vary the concentrations of the doping gases in order to obtain a variation of the index between successive layers. Then the tube is collapsed to form the preform.
It is therefore readily apparent that the process is slow and as a result of the heating point being displaced along the tube, it is necessary to wait until the tube temperature is sufficiently high for the chemical reaction to be started in the following zone, whilst in addition the efficiency of the process is very poor. It has in fact been estimated that between 70 and 75% of the products of the reaction are entrained towards the outlet of the tube, whilst only about 25% of these products are deposited on the tube walls.